This invention relates to a method of manufacturing of optical devices, and in particular, though not exclusively, to manufacturing integrated optical devices or optoelectronic devices, for example, semiconductor optoelectronic devices such as laser diodes, optical modulators, optical amplifiers, optical switches, optical detectors, and the like. The invention further relates to Optoelectronic Integrated Circuits (OEICs) and Photonic Integrated Circuits (PICs) including such devices.
The invention particularly, though not exclusively, relates to a method of manufacturing an optical device using a new and improved impurity induced Quantum Well Intermixing (QWI) process.
Monolithic integration of different optical components onto a single epitaxial layer is highly desirable in optical communication systems. One of the fundamental demands for monolithic integration is to realise different semiconductor band-gaps within one epitaxial layer. For example, a 2xc3x972 cross-point switch incorporating semiconductor optical amplifiers, passive waveguide splitters, and electro-absorption (EA) modulators typically requires three band-gaps. The operation wavelength for the switches, and therefore for the amplifiers, is typically 1.55 xcexcm, but a much wider band-gap is required for the passive waveguides in order to minimize the absorption of light propagation along the waveguides. Moreover, the optimum absorption band-gap for the EA modulators is around 20-50 nm shorter than that of the amplifiers, to realise a low insertion loss and high extinction ratio. Multiband-gap energy structures also find applications in devices such as multiwavelength sources in WDM systems and photodetectors.
Many techniques are currently under investigation for such a purpose. Although those based on selective regrowth appear promising, expensive facilities such as Metal-Organic Chemical Vapor Deposition (MOCVD) are needed during the entire production process, and two-dimensional patterning of the band-gap is not possible. Other approaches are based on Quantum Well Intermixing (QWI).
Quantum Well Intermixing (QWI) is a process which has been reported as providing a possible route to monolothic optoelectronic integration. QWI may be performed in III-V semiconductor materials, eg Aluminium Gallium Arsenide (AlGaAs) and Indium Gallium Arsenide Phosphide (InGaAsP), which may be grown on binary substrates, eg Gallium Arsenide (GaAs) or Indium Phosphide (InP). QWI alters the band-gap of an as-grown structure through interdiffusion of elements of a Quantum Well (QW) and associated barriers to produce an alloy of the constituent components. The alloy has a band-gap which is larger than that of the as-grown QW. Any optical radiation (light) generated within the QW where no QWI has taken place can therefore pass through a QWI or xe2x80x9cintermixedxe2x80x9d region of alloy which is effectively transparent to the said optical radiation.
Various QWI techniques have been reported in the literature. For example, QWI can be performed by high temperature diffusion of elements such as Zinc into a semiconductor material including a QW.
QWI can also be performed by implantation of elements such as silicon into a QW semiconductor material. In such a technique the implantation element introduces point defects in the structure of the semiconductor material which are moved through the semiconductor material inducing intermixing in the QW structure by a high temperature annealing step.
Such QWI techniques have been reported in xe2x80x9cApplications of Neutral Impurity Disordering in Fabricating Low-Loss Optical Waveguides and Integrated Waveguide Devicesxe2x80x9d, Marsh et al, Optical and Quantum Electronics, 23, 1991, s941-s957, the content of which is incorporated herein by reference.
A problem exists with such techniques in that, although the QWI will alter (increase) the band-gap of the semiconductor material post-growth, residual diffusion or implantation dopants can introduce large losses due to the free carrier absorption coefficient of these dopant elements.
A further reported QWI technique providing intermixing is Impurity Free Vacancy Diffusion (IFVD). When performing IFVD the top cap layer of the III-V semiconductor structure is typically GaAs or Indium Gallium Arsenide (InGaAs). Upon the top layer is deposited a Silica (SiO2) film. Subsequent rapid thermal annealing of the semiconductor material causes bonds to break within the semiconductor alloy and Gallium ions or atoms, which are susceptible to Silica (SiO2), to dissolve into the Silica so as to leave vacancies in the cap layer. The vacancies then diffuse through the semiconductor structure inducing layer intermixing, eg in the QW structure.
IFVD has been reported in xe2x80x9cQuantitative Model for the Kinetics of Composition Intermixing in GaAsxe2x80x94AlGaAs Quantum xe2x80x9cConfined Heterostructuresxe2x80x9d, by Helmy et al, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4, No. 4, July/August 1998, pp 653-660, the content of which is incorporated herein by reference.
Reported QWI, and particularly IFVD methods, suffer from a number of disadvantages, eg the temperature at which Gallium out-diffuses from the semiconductor material to the Silica (SiO2) film.
It is an object of at least one aspect of the present invention to obviate or at least mitigate at least one of the aforementioned disadvantages/problems in the prior art.
It is also an object of at least one aspect of the present invention to provide an improved method of manufacturing an optical device using an improved QWI process.
According to a first aspect of the present invention there is provided a method of manufacturing an optical device, a device body portion from which the device is to be made including at least one Quantum Well (QW), the method including the step of:
causing an impurity material to intermix with the at least one Quantum Well, wherein the impurity material at least includes Copper (Cu).
The impurity material may substantially comprise Copper or an alloy thereof.
It has surprisingly been found that Copper diffuses around 106 times faster than previously used impurities such as Zinc (Zn).
Preferably the method includes a preceding step of depositing on or adjacent the device body portion a layer including the impurity material.
In a first embodiment the impurity material may be incorporated with a carrier material. The carrier may be a dielectric material such as Silica (SiO2) or Aluminum Oxide (Al2O3). In such case the layer may be deposited directly upon a surface of the device body portion, eg by sputtering.
In this first embodiment, the layer may be deposited by use of a diode or magnetron sputterer.
In a second embodiment the layer may comprise a layer of the impurity material which may be deposited adjacent a surface of the device body portion upon a spacer layer. The spacer layer may comprise a dielectric material such as Silica (SiO2) or Aluminum Oxide (Al2O3).
A further layer, eg a further dielectric layer may be deposited on the layer.
In this second embodiment, the layer may be deposited by use of sputtering and the spacer layer and optional further layer may be deposited by use of sputtering or another technique, eg PECVD.
Preferably the method also includes the yet further preceding steps of:
providing a substrate;
growing on the substrate:
a first optical cladding layer;
a core guiding layer including the at least one Quantum Well (QW);
a second optical cladding layer; and
optionally a contact layer.
The first optical cladding layer, core guiding layer, second optical cladding layer and contact layer may be grown by Molecular Beam Epitaxy (MBE) or Metal Organic Chemical Vapour Deposition (MOCVD).
In a modification to the first embodiment the layer may be removed from the device body portion prior to intermixing.
Preferably the impurity material is caused to intermix with the at least Quantum Well (QW) by raising the device body portion to an elevated temperature for a predetermined time.
The elevated temperature may be in the region 700xc2x0 C. to 950xc2x0 C., while the predetermined time may be in the region of 30 seconds to 300 seconds.
The step of raising the device body portion to an elevated temperature may comprise annealing of the device body portion, which causes diffusion into the at least one Quantum Well of impurity material and out diffusion of ions or atoms from the Quantum Wells to the carrier material or spacer layer.
Preferably the method includes the step of:
causing the impurity material to diffuse into the device body portion and also material (eg ions or atoms) of the device body portion to diffuse out and into a further material.
This therefore advantageously combines impurity induced and impurity free intermixing.
In one embodiment the further material may be a dielectric material such as Silica (SiO2) or Aluminum Oxide (Al2O3).
Preferably the method includes the steps of:
patterning a surface of the device body portion with a plurality of areas of the impurity material, at least two of the areas of impurity material being spaced from the surface by different amounts;
causing the impurity material of the plurality of areas to intermix with the at least one Quantum Well so as to tune a band-gap of the intermixed at least one Quantum Well in the at least two areas to different values.
According to a second aspect of the present invention there is provided an optical device fabricated from a method according to the first aspect of the present invention.
The optical device may be an integrated optical device or an optoelectronic device.
The device body portion may be fabricated in a III-V semiconductor materials system.
The III-V semiconductor materials system may be a Gallium Arsenide (GaAs) based system, and may operate at a wavelength or wavelengths of substantially between 600 nm and 1300 nm. Alternatively, the III-V semiconductor materials system may be an Indium Phosphide based system, and may operate at a wavelength or wavelengths of substantially between 1200 nm and 1700 nm. The device body portion may be made at least partly from Aluminium Gallium Arsenide (AlGaAs), Indium Gallium Arsenide (InGaAs), Indium Gallium Arsenide Phosphide (InGaAsP), Indium Gallium Aluminium Arsenide (InGaAlAs) and/or Indium Gallium Aluminum Phosphide (InGaAlP).
The device body portion may comprise a substrate upon which are provided a first optical cladding layer, a core guiding layer, and a second optical cladding layer and optionally a contact layer.
At least one Quantum Well (QW) may be provided within the core guiding layer.
Alternatively, or additionally, at least one Quantum Well (QW) may be provided within one or both of the cladding layers. It will be appreciated by the reader that in the latter case one is likely more interested in tuning the refractive index rather than the band-gap of the cladding layer(s).
The core guiding layer, as-grown, may have a smaller band-gap and higher refractive index than the first and second optical layers.
According to a third aspect of the present invention there is provided an optical integrated circuit, optoelectronic integrated circuit (OEIC), or photonic integrated circuit (PIC) including at least one optical device according to the second aspect of the present invention.
According to a fourth aspect of the present invention there is provided a device body portion (xe2x80x9csamplexe2x80x9d) when used in a method according to the first aspect of the present invention.
According to a fifth aspect of the present invention there is provided a wafer of material including at least one device body portion when used in a method according to the first aspect of the present invention.